In recent years, electronic devices such as mobile phones and digital cameras have been drastically reduced in size and increased in processing speed. This has raised demands for smaller size or higher processing speed for components such as semiconductor devices, to be installed in electronic devices.
In a conventional semiconductor device, a semiconductor chip and a bonding wire connecting the semiconductor chip to an external equipment are covered by sealing resin, and an outer lead or the like is used to connect the semiconductor chip to the external equipment (substrates of electronic devices). With this structure, it is difficult to fully meet the demands for smaller size and higher processing speed.
In view of the foregoing circumstance, a bare chip assembly, in which a bare chip is directly mounted on a substrate, has been employed to meet the demands. In the bare chip assembly, a solder or an external connection terminal, made of gold or the like, is formed on an electrode pad of a semiconductor chip and bonded to a land formed on the substrate.
The bare chip assembly, however, has problems. Specifically, the semiconductor chip is not sufficiently protected and therefore is difficult to handle. Further, the external connection terminal easily cracks due to change in temperature in the environment. These problems occur owning to the following reasons. An outer lead having spring effect is not employed as the external connection terminal. Thus, owning to the difference in coefficient of linear expansion between a substrate and a semiconductor chip, stress is concentrated on a section connecting the external connection terminal, which section is in between the substrate and the semiconductor chip.
Japanese Unexamined Patent Publication No. 145174/1999 (Tokukaihei 11-145174) (published on May 28, 1999) suggests means for solving the problem of breakage of an external connection terminal due to stress concentration. Concretely, the publication suggests a semiconductor device including an atom mixed layer, and a method for manufacturing the semiconductor device.
The following describes the semiconductor device of publication No. 145174/1999, with reference to FIG. 14. The semiconductor device is manufactured by use of the bare chip assembly. FIG. 14 is an enlarged view showing the vicinity of the external connection terminal of the semiconductor device. As shown in FIG. 14, a passivation film 103 made of SiN is formed on a surface of the semiconductor chip 100, on which surface a semiconductor element is to be mounted. The passivation film 103 has an opening section over an electrode pad 102. On the passive film 103, a polyimide film 104 is formed to protect the electronic pad, etc., against external damage. The polyimide film 104 has an opening section inside of the opening section 103a of the passivation film 103.
Further, the following films are layered to form a base metal layer of the bump: a Cr sputtered film (0.1 μm), which adheres well to the electrode pad 102; a Cu sputtered film (0.1 μm), which is an external connection terminal 131 and adheres well to a Sn—Pb solder bump; and a Au sputtered film (1 μm), which is an oxidization preventing film. Publication No. 145174/1999 proposes the method for preventing the cracking and peeling-off problems when adopting Pb—Sn alloy, which is a common solder material.
Specifically, after the Cu sputtered film is formed, or after the Au sputtered film is formed, Ar+ ions are applied to form the atom mixed layer of approximately 0.05 μm in between the Cr film and the Cu film.
Publication No. 145174/1999 reported a result of a high-temperature storage test performed at 150° C. on the semiconductor device having the atom mixed layer. According to the result of the test, cracking and peeling-off problems were not observed. Specifically, Sn contained in the solder that is the external connection terminal 131 becomes alloyed with Cu contained in the Cu film, so that a non-reacted part of the Cu film disappears. Thus, a Pb precipitate grown widely occupies an interface between the Sn—Cu alloy layer and the Cr film. However, the atom mixed layer adheres well to the Pb precipitate. Therefore, there is no problem of cracking and peeling-off.
With this conventional structure, however, the polyimide film 104 is formed on the electrode pad metal 102, whose adhesive force is not sufficiently large, and the metal film which serves as the external connection terminal is formed in this area. Further, the metal film, except for the section bonded to the electrode pad metal 102, is formed so as to adhere only to the polyimide film 104, whose adhesive force is not sufficiently strong. Thus, the method of providing the atom mixed layer as suggested in the above Publication not sufficient to prevent the problems of peeling-off and cracking of the metal film constituting the external connection terminal and the interface of the metal film due to the stress concentrated on the metal film which serves as an external connection terminal and the interface of the metal film.